Coherent optical receiver, apparatus and method for detecting interchannel skew in coherent optical receiver

ABSTRACT

In a coherent optical receiver, sufficient demodulation becomes impossible and consequently receiving performance deteriorates if an interchannel skew arises, therefore, a coherent optical receiver according to an exemplary aspect of the invention includes a local light source; a 90-degree hybrid circuit; an optoelectronic converter; an analog-to-digital converter; and a digital signal processing unit, wherein the 90-degree hybrid circuit makes multiplexed signal light interfere with local light from the local light source, and outputs a plurality of optical signals separated into a plurality of signal components; the optoelectronic converter detects the optical signals and outputs detected electrical signals; the analog-to-digital converter quantizes the detected electrical signals and outputs quantized signals; and the digital signal processing unit includes a skew compensation unit for compensating a difference in propagation delay between the plurality of signal components, and a demodulation unit for demodulating the quantized signals.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a National Stage of International Application No.PCT/JP2011/061601 filed May 13, 2011, claiming priority based onJapanese Patent Application No. 2010-116878 filed May 21, 2010, thecontents of all of which are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present invention relates to coherent optical receivers, andapparatuses and methods for detecting interchannel skew in a coherentoptical receiver, and, in particular, to a coherent optical receiverwhich receives polarization multiplexing optical signals by means ofcoherent detection and digital signal processing, and to an apparatusand a method for detecting interchannel skew in the coherent opticalreceiver.

BACKGROUND ART

The data capacity in the networks has been increasing year by year dueto the wide spread of the Internet. In the trunk line connectingmetropolitan areas, the optical transmission link whose transmissioncapacity per one channel is 10 Gb/s or 40 Gb/s has already beenintroduced. The On-Off-keying (OOK) is employed as a modulation schemein 10 Gb/s transmission. Although the OOK scheme is also employed in 40Gb/s transmission, it is unsuitable for long-haul transmission becausethe transmission characteristics are greatly influenced by the chromaticdispersion due to the narrow optical pulse width of 25 ps. Therefore,the multilevel modulation scheme using phase modulation and thepolarization multiplexing scheme have been adopted, and the DualPolarization Quadrature Phase Shift Keying (DP-QPSK) scheme is mainlyemployed for 100 Gb/s class transmission systems.

The optical signals modulated by DP-QPSK scheme in a transmitter arereceived and demodulated by a coherent optical receiver (refer to NonPatent Literature 1, for example). FIG. 12 shows an example of theconfiguration of a related coherent optical receiver. The relatedcoherent optical receiver 600 has a local light source 610, a 90-degreehybrid circuit (90° HYBRID) 620, an optoelectronic converter (O/E) 630,an analog-to-digital converter (ADC) 640, and a digital signalprocessing unit (DSP) 650.

Signal light and local light can be represented by the followingformulae as a single polarization signal, respectively.S(t)=exp[jωt]  (1)L(t)=exp[j(ω+Δω)t]  (2)

Here, Δω represents a frequency offset between the signal light and thelocal light. The signal light and the local light are inputted into the90-degree hybrid circuit (90° HYBRID) 620, passing through an opticalinterference system, and converted into electric signals by theoptoelectronic converters (O/E) 630, each of which is composed ofdifferentially configured photodiodes. At this time, the outputsrepresented by the following formulae (3) and (4) are obtained from anI_(x) port and a Q_(X) port, respectively.I _(X)(t)=cos(Δωt)  (3)Q _(X)(t)=sin(Δωt)  (4)

In the case of a polarization multiplexed signal, the signal light S (t)is expressed as S (t)=E_(X)+E_(Y), the cosine components of the mixedsignal of E_(X)+E_(Y) are outputted from I_(X) port and I_(Y) port, andthe sine components of the mixed signal of E_(X)+E_(Y) are outputtedfrom Q_(X) port and Q_(Y) port.

The signals outputted from each port are AD converted by theanalog-to-digital converters (ADC) 640, and then inputted into thedigital signal processing unit (DSP) 650. The digital signal processingunit (DSP) 650 demultiplexes inputted signals to E_(X) signal and E_(Y)signal by polarization demultiplexing processing, and demodulates thesignals of E_(X) and E_(Y) into four levels by phase estimationprocessing.

In this way, the DP-QPSK signals can be demodulated by using thecoherent optical receiver.

-   Non Patent Literature 1: M. G. Taylor, “Coherent Detection Method    Using DSP for Demodulation of Signal and Subsequent Equalization of    Propagation Impairments”, IEEE Photonics Technology Letters, vol.    16, No. 2, February 2004, p. 674-676

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

The expressions of the signals in the above-described formulae (3) and(4) are true only if, in the coherent optical receiver 600, all lengthsof four signal lines from the outputs of the 90-degree hybrid circuit620 to the inputs of the analog-to-digital converter 640 are equal.However, it is difficult to make precisely equal the length betweenthose four channels, that is, the length of optical fiber cables fromthe output of the 90-degree hybrid circuit 620 to the inputs of theoptoelectronic converters 630, and the length of coaxial wires from theoutputs of the optoelectronic converters 630 to the inputs of theanalog-to-digital converters 640.

Here, if the lengths of the lines are not equal between those fourchannels, there arises a delay in signal transmission, that is, a skew.The influence of the skew will be described referring to FIG. 13. FIG.13 is a block diagram showing the configuration of related 90-degreehybrid circuits 620 and their peripherals. In this figure, “PBS”represents a polarization beam splitter, “CPL” represents an opticalcoupler, “τ” represents a 90-degree phase difference unit, and “BR”represents a balanced photodetector as the optoelectronic converter(O/E) 630, respectively.

If there exists a skew T in the channel 2 (CH2) against the channel 1(CH1), the above-described formula (4) is changed into the followingformula (5).Q _(X)(t)=sin(Δω(t+T))  (5)

In the absence of the above-described skew T, the polarizationdemultiplexing and the phase estimation can be performed by the digitalsignal processing using the above-described formulae (3) and (4), anddemodulation can be achieved perfectly. However, if there exists aninterchannel skew, the output signal from the port Q_(X) expressed bythe formula (4) changes into the output signal expressed by the formula(5), and the demodulation becomes imperfect even though the digitalsignal processing is performed, and thus sufficient performance can notbe achieved. As described above, in a coherent optical receiver, thereis a problem that sufficient demodulation becomes impossible andconsequently receiving performance deteriorates if an interchannel skewarises.

The object of the present invention is to provide a coherent opticalreceiver and an apparatus and a method for detecting interchannel skewin the coherent optical receiver which solve the problem mentioned abovethat in a coherent optical receiver, sufficient demodulation becomesimpossible and consequently receiving performance deteriorates if aninterchannel skew arises.

Means for Solving a Problem

A coherent optical receiver according to an exemplary aspect of theinvention includes a local light source; a 90-degree hybrid circuit; anoptoelectronic converter; an analog-to-digital converter; and a digitalsignal processing unit, wherein the 90-degree hybrid circuit makesmultiplexed signal light interfere with local light from the local lightsource, and outputs a plurality of optical signals separated into aplurality of signal components; the optoelectronic converter detects theoptical signals and outputs detected electrical signals; theanalog-to-digital converter quantizes the detected electrical signalsand outputs quantized signals; and the digital signal processing unitincludes a skew compensation unit for compensating a difference inpropagation delay between the plurality of signal components, and ademodulation unit for demodulating the quantized signals.

An apparatus for detecting interchannel skew in a coherent opticalreceiver according to an exemplary aspect of the invention includes acoherent optical receiver; a test light source; an analog-to-digitalconverter; an FFT operation unit; and a control block; wherein thecoherent optical receiver includes a local light source, a 90-degreehybrid circuit, and an optoelectronic converter; the 90-degree hybridcircuit makes test light from the test light source interfere with locallight from the local light source, and outputs a plurality of opticalsignals separated into a plurality of signal components; theoptoelectronic converter detects the optical signals and outputsdetected electrical signals; the analog-to-digital converter quantizesthe detected electrical signals and outputs quantized signals; the FFToperation unit performs a fast Fourier transform process on thequantized signals; and the control block calculates a difference inpropagation delay between the plurality of signal components from theresults of the fast Fourier transform process.

A method for detecting interchannel skew in a coherent optical receiveraccording to an exemplary aspect of the invention includes the steps of:outputting a plurality of optical signals separated into a plurality ofsignal components by making test light from a test light sourceinterfere with local light from a local light source; detecting theoptical signals and outputting detected electrical signals; quantizingthe detected electrical signals and outputting quantized signals;performing a fast Fourier transform process on the quantized signals;and calculating a difference in propagation delay between the pluralityof signal components from the results of the fast Fourier transformprocess.

Effect of the Invention

According to the coherent optical receiver by the present invention,even if a skew arises between the channels, it becomes possible toachieve sufficient demodulation and thus to suppress deterioration ofreception performance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the configuration of a coherentoptical receiver in accordance with the first exemplary embodiment ofthe present invention.

FIG. 2 is a block diagram showing the configuration of an apparatus fordetecting interchannel skew in the coherent optical receiver inaccordance with the first exemplary embodiment of the present invention.

FIG. 3 is a flowchart illustrating a method for detecting interchannelskew in the coherent optical receiver in accordance with the firstexemplary embodiment of the present invention.

FIG. 4 is a diagrammatic illustration where FFT data are plotted againstpoint number, which are derived by an FFT operation unit in the coherentoptical receiver in accordance with the first exemplary embodiment ofthe present invention.

FIG. 5 is a diagrammatic illustration plotting the relations betweenphase difference and angular frequency at Q_(X) port and I_(Y) port ofthe coherent optical receiver in accordance with the first exemplaryembodiment of the present invention.

FIG. 6 is a flowchart illustrating another method for detectinginterchannel skew in the coherent optical receiver in accordance withthe first exemplary embodiment of the present invention.

FIG. 7 is a diagrammatic illustration plotting another set of therelations between phase difference and angular frequency at Q_(X) portand I_(Y) port of the coherent optical receiver in accordance with thefirst exemplary embodiment of the present invention.

FIG. 8 is a block diagram showing the configuration of an apparatus fordetecting interchannel skew in the coherent optical receiver inaccordance with the second exemplary embodiment of the presentinvention.

FIG. 9 is a diagrammatic illustration where FFT data are plotted againstpoint number, which are derived by an FFT operation unit in the coherentoptical receiver in accordance with the second exemplary embodiment ofthe present invention.

FIG. 10 is a block diagram showing the configuration of an apparatus fordetecting interchannel skew in the coherent optical receiver inaccordance with the third exemplary embodiment of the present invention.

FIG. 11 is a flowchart illustrating a method for detecting interchannelskew in the coherent optical receiver in accordance with the thirdexemplary embodiment of the present invention.

FIG. 12 is a block diagram showing the configuration of a relatedcoherent optical receiver.

FIG. 13 is a block diagram showing the configuration of related90-degree hybrid circuits and their peripherals.

DESCRIPTION OF EMBODIMENTS

The exemplary embodiments of the present invention will be describedwith reference to drawings below.

The First Exemplary Embodiment

FIG. 1 is a block diagram showing the configuration of a coherentoptical receiver 100 in accordance with the first exemplary embodimentof the present invention. The coherent optical receiver 100 has a locallight source 110, a 90-degree hybrid circuit (90° HYBRID) 120,optoelectronic converters (O/E) 130, analog-to-digital converters (ADC)140, and a digital signal processing unit (DSP) 150.

The 90-degree hybrid circuit (90° HYBRID) 120 makes multiplexed signallight (SIGNAL) interfere with the local light from the local lightsource 110, and outputs a plurality of optical signals separated intorespective signal components. In the present exemplary embodiment, thecase will be described in which DP-QPSK modulation scheme is used.Accordingly, the 90-degree hybrid circuit (90° HYBRID) 120 outputsfour-wave light signals including four-channel signal componentsrespectively which are composed of in-phase components (I_(X), I_(Y))and quadrature-phase components (Q_(X), Q_(Y)) for each of twopolarizations (X polarization and Y polarization).

The optoelectronic converter (O/E) 130 detects the respective lightsignals outputted from the 90-degree hybrid circuit 120, and outputs thedetected electrical signals. The analog-to-digital converter (ADC) 140quantizes the detected electrical signals, and outputs the quantizedsignals.

The digital signal processing unit (DSP) 150 is provided with a skewcompensation unit 151 which compensates the difference in propagationdelay between a plurality of signal components (hereinafter, referred toas “skew”), and a demodulation unit 152. The skew compensation unit 151can be configured by using an FIR (Finite Impulse Response) filter andthe like, for example; and in such a case, it holds filter coefficientswhich are determined on the basis of a skew value. The demodulation unit152 separates the quantized signals into X polarization signals and Ypolarization signals by the polarization demultiplexing process, andthen demodulates each of the four-channel signal components by the phaseestimation process.

Next, a method for detecting interchannel skew in the coherent opticalreceiver 100 will be described, referring to FIG. 2. In the following,the case will be described in which the digital signal processing unit(DSP) 150 in the coherent optical receiver 100 is provided with a bufferunit (BUF) 153 and an FFT operation unit (FFT) 154. Here, the FFToperation unit 154 performs a fast Fourier transform (referred to as“FFT”, hereinafter) process on the quantized signals outputted by theanalog-to-digital converter 140. In FIG. 2, illustrations of the skewcompensation unit 151 and the demodulation unit 152 are omitted.

In the following, first, the case will be described in which there is a90-degree error between I port and Q port in the 90-degree hybridcircuit. That is, although there is a delay corresponding to the signalcycle of 90 degree between I port and Q port in the 90-degree hybridcircuit, the phase difference does not necessarily correspond to 90degree exactly due to the variability in the manufacturing process ofthe 90-degree hybrid circuit. Taking into account a delay Δτ due to theerror in the 90-degree phase difference, the formula (5) described aboveis changed into the following formula (6).Q _(X)(t)=sin(Δω(t+T)+Δτ)  (6)When there exists this 90-degree error, the output signal from the portQ_(X) expressed by the formula (4) changes into the output signalexpressed by the formula (6), and also in this case, the demodulationbecomes insufficient even though digital signal processing is performed,and thus sufficient performance cannot be achieved.

As shown in FIG. 2, a test light source 170 and a control block 180 areconnected to the coherent optical receiver 100, and thereby an apparatusfor detecting interchannel skew in the coherent optical receiver 1000 isconfigured. The control block 180 includes a control unit 181, a memoryunit 182, and an operational processing unit 183. The operationalprocessing unit 183 is provided with a peak detection unit 184 and askew calculation unit 185, and calculates a skew value from FFTprocessed results. Here, the peak detection unit 184 and the skewcalculation unit 185 can be configured by specific signal processingcircuits, and may also be configured by a central processing unit (CPU)and programs for enabling the CPU to execute a process.

The test light source (TEST) 170 is connected to a signal port 121 ofthe 90-degree hybrid circuit (90° HYBRID) 120, and the local lightsource 110 is connected to a local port 122. Light components outputtedfrom the I_(X), Q_(X), I_(Y), and Q_(Y) ports, which are output ports ofthe 90-degree hybrid circuit (90° HYBRID) 120, are inputted into theoptoelectronic converters (O/E) 130, respectively.

In detecting interchannel skew in the coherent optical receiver 100,first, a continuous wave (CW) light as a test light with a frequencyf_(S) (its wavelength is equal to λ_(S)) is inputted from the test lightsource 170 into the signal port 121. Here, a wavelength tunable lightsource can be used for the test light source 170. On the other hand, aCW light as a local light of a frequency f_(O), (its wavelength is equalto λ_(O)) is inputted from the local light source 110 into the localport 122. The test light of frequency fs and the local light offrequency f_(O) interfere in the 90-degree hybrid circuit 120, and beatsignals of a frequency f_(IF)=|f_(S)−f_(O)| are outputted. Here, thebeat signals outputted from the I_(X), Q_(X), I_(Y), and Q_(Y) ports arerepresented by the following formulae form (7) to (10), respectively.I _(X)=cos(2πf _(IF) t+φ _(IX))  (7)Q _(X)=sin(2πf _(IF) t+φ _(QX))  (8)I _(Y)=cos(2πf _(IF) t+φ _(IY))  (9)Q _(Y)=sin(2πf _(IF) t+φ _(QY))  (10)

These beat signals are converted into electrical signals by theoptoelectronic converters (O/E) 130, quantized by the analog-to-digitalconverters (ADC) 140, and then inputted into the digital signalprocessing unit (DSP) 150, respectively. In the digital signalprocessing unit (DSP) 150, the signals are divided into blocks withrespect to each predetermined processing unit (4096 bits, for example)by buffer units 153, and subjected to an FFT process in the FFToperation unit (FFT) 154. As a result, each of matrices I^_(X)(N),Q^_(X)(N), I^_(Y)(N), and Q^_(Y)(N) is obtained as each output of theFFT operation unit 154. Here, “N” represents a point number of FFT andit is equal to a value from 0 to 4095, for example.

Next, the method for detecting interchannel skew in the coherent opticalreceiver in accordance with the present exemplary embodiment will bedescribed referring to the flowchart shown in FIG. 3. First, thefrequency of the test light source 170 is set at a frequency f_(S1) (itswavelength is equal to λ_(S1)) (step S1). Accordingly, a beat signal ofa frequency f_(IF)=|f_(S1)−f_(O)| is outputted from each output port ofthe 90-degree hybrid circuit (90° HYBRID) 120.

Next, data capturing process is started (step S2). At that time, thecontrol unit 181 in the control block 180 transmits a data capturesignal to the digital signal processing unit (DSP) (step S3). The FFToperation unit 154 receives the data capture signal, triggered by thesignal, it performs an FFT process on the data stored in the buffer unit(BUF) 153 at that time (step S4), and returns FFT data I^_(X)(N),Q^_(X)(N), I^_(Y)(N), and Q^_(Y)(N) to the control unit 181. The controlunit 181 stores the acquired FFT data in the memory unit 182 (step S5).

By an instruction from the control unit 181, the peak detection unit 184in the operational processing unit 183 extracts the data I^_(X)(N_(max))having the maximum magnitude from 4096 points of the FFT data I^_(X)(N).The frequency (peak frequency) f_(max) and the phase (peak phase)φ_(max) at that point are derived by calculation (step 6). In FIG. 4, adiagrammatic illustration is shown where I^_(X)(N) are plotted againstpoint number N. Here, since the FFT data I^_(X)(N) are composed ofcomplex numbers, the vertical axis of the figure represents themagnitude of I^_(X)(N), |I^_(X)(N)|, and the horizontal axis representsthe point number N in the FFT data. As shown in FIG. 4, if |I^_(X)(N)|has a peak value at the point number N_(max), the peak detection unit184 detects the I^_(X)(N_(max)). Here, f_(T) representing a samplingfrequency in the analog-to-digital converters (ADC) 140, a frequencyinterval of the FFT process is equal to f_(T)/4096. Therefore, the peakfrequency f_(max) at the peak of I^_(X)(N) is equal to N_(max)f_(T)/4096. And then, peak phase information φ_(max)=∠(I^_(X)(N_(max)))is calculated by using the FFT data I^_(X)(N_(max)) at the peakfrequency F_(max).

In this way, the peak detection unit 184 derives the peak frequencyf_(max) and the peak phase φ_(max) at the peak of the magnitude of theFFT data I^_(X)(N), and the control unit 181 stores them in the memoryunit 182 as a frequency f_(IX(1,1)) and a phase φ_(IX(1,1)) (step S7).At that time, the other data of the FFT data I^_(X)(N) can beeliminated.

In order to reduce the influence of a measurement error, the processesfrom step 3 to step 7 are repeated n times, and frequencies f_(IX(1,n))and phases φ_(IX(1,n)) are stored in the memory unit 182, respectively(feedback loop FB1). When the n-th loop has been completed, an endingflag is set (step 8).

Next, after changing the frequency of the test light source 170 into afrequency f_(S2) (step S9), the processes from step 2 to step 7 arerepeated again, and then frequencies f_(IX(2,n)) and phases φ_(IX(2, n))are stored in the memory unit 182 (step S7). When detecting the endingflag (step S8), the frequency of the test light source 170 is furtherswept (step S9), and then the processes from step 2 to step 8 arerepeated again (feedback loop FB2). By repeating the feedback loop FB2 mtimes, frequencies f_(IX(m,n)) and phases φ_(IX(m,n)) are stored in thememory unit 182, respectively. By performing similar processes forQ^_(X)(N), I^_(Y)(N), and Q^_(Y)(N), frequencies f_(QX(m,n)),f_(IY(m,n)), and f_(QY(m,n)), and phases φ_(QX(m,n)), φ_(IY(m, n)), andφ_(QY(m, n)) are stored in the memory unit 182, respectively.

When the above-mentioned processes have been completed, by aninstruction from the control unit 181, the skew calculation unit 185 inthe operational processing unit 183 calculates skews (step 10). Forexample, using the I_(X) port as a reference, a skew in the I_(X) portbecomes zero, and a skew in each of the ports Q_(X), I_(Y), and Q_(Y) isrepresented by phase lead or phase lag against the I_(x) port.Specifically, first, the phase differences in the respective ports areobtained for a measurement cycle number n and a measurement frequency mby calculating the following quantities, respectively.φ_(IX(m,n))=0φ_(QX(m,n))−φ_(IX(m, n))φ_(IY(m,n))−φ_(IX(m, n))φ_(QY(m,n))−φ_(IX(m, n))

FIG. 5 shows a diagrammatic illustration plotting the relations betweeneach phase difference of φ_(QX-IX) and φ_(IY-IX) in the Q_(X) port andI_(Y) port using the I_(X) port as a reference and the angular frequency2πf_(max). By using this figure, approximation formulae represented bylinear functions are derived for the Q_(X) port and I_(Y) portrespectively, as follows.φ_(QX-IX) =T ₁(2πf)+φ₁φ_(IY-IX) =T ₂(2πf)+φ₂An approximation formula is similarly derived for the Q_(Y) port asfollows.φ_(QY-IX) =T ₃(2πf)+φ₃

Each gradient of T₁, T₂, and T₃ obtained here represents a skew againstthe I_(X) port. Here, the accuracy in the skew detection is obtained bythe relation between the point number N of the FFT process and f_(IF).For example, since a period is equal to 1 ns (=1000 ps) with f_(IF)being equal to 1 GHz, the accuracy of the skew detection becomes equalto 0.24 ps (=1000/4096) when N is equal to 4096. That is, it is foundthat the detection accuracy deteriorates with f_(if) decreasing.

On the other hand, a phase difference in the Q_(Y) port against theI_(Y) port is represented as follows.φ_(QY(m,n))−φ_(IY(m, n))Here, the relation to the angular frequency 2πf_(max) is approximatelyrepresented by the following linear function, as is the case mentionedabove.φ_(QY-IY) =T ₄(2πf)+φ₄

Since each of the phase differences φhd QX-IX and φ_(QY-IY) is equal toπ/2 without frequency offset, each of φ₁ and φ₄ should become π/2.Therefore, the 90-degree error between the I_(X) port and the Q_(X)port, and that between the I_(Y) port and the Q_(Y) port become φ₁−π/2and φ₄−π/2, respectively. Accordingly, by deriving φ₁ and φ₄ from they-intercepts of the linear functions shown in FIG. 5, the 90-degreeerrors in the I_(Y) port and the Q_(Y) port are obtained.

As mentioned above, according to the apparatus and the method fordetecting interchannel skew in the coherent optical receiver of thisexemplary embodiment, it becomes possible to calculate skews between theoutput ports and the 90-degree errors between the I port and the Q port.That is to say, it is possible to input test light into the signal portof the 90-degree hybrid circuit, observe beat signals between the testlight and the local light by means of analog-to-digital converters, andcalculate the skews and the 90-degree errors by using the phaseinformation obtained by performing an FFT operation.

Furthermore, according to the coherent optical receiver 100 of thepresent exemplary embodiment, by compensating the skew values obtainedabove in the skew compensation unit 151 of the digital signal processingunit 150, it becomes possible to demodulate sufficiently even thoughthere arises a skew between the channels, and suppress the degradationof receiving performance.

In the above-mentioned exemplary embodiment, the 90-degree error betweenI port and Q port is calculated by obtaining the difference in peakphase between the channels at each of the frequencies with sweeping thefrequency of the test light source as shown by the feedback loop FB2 inFIG. 3. However, if the 90-degree error can be neglected, it is possibleto detect an inter-channel skew more simply.

FIG. 6 shows a flowchart of the method for detecting interchannel skewfor this case. First, a frequency of the test light source 170 is set ata frequency f_(S1) (its wavelength is equal to λ_(S1)) (step S1).Accordingly, the beat signal of the frequency f_(IF)=|f_(S1)−f_(O)| isoutputted from each output port of the 90-degree hybrid circuit (90°HYBRID) 120.

Next, data capturing process is started (step S2). At that time, thecontrol unit 181 in the control block 180 transmits a data capturesignal to the digital signal processing unit (DSP) 150 (step S3). TheFFT operation unit 154 receives the data capture signal, triggered bythe signal, it performs an FFT process on the data stored in the bufferunits (BUF) 153 at that time (step S4), and returns FFT data I^_(X)(N),Q^_(X)(N), I^_(Y)(N), and Q^_(Y)(N) to the control unit 181. The controlunit 181 stores the acquired FFT data in the memory unit 182 (step S5).

By an instruction from the control unit 181, the peak detection unit 184in the operational processing unit 183 extracts the data I^_(X)(N_(max)having the maximum magnitude max, from 4096 points of the FFT dataI^_(X)(N). The frequency (peak frequency) f_(max) and the phase (peakphase) φ_(max) at that point are derived by calculation (step S6). Thecontrol unit 181 stores this peak frequency and this peak phase in thememory unit 182 as a frequency f_(IX(1)) and a phase φ_(IX(1)) (stepS7).

In order to reduce the influence of a measurement error, the processesfrom step 3 to step 7 are repeated n times, and frequencies f_(IX(n))and phases φ_(IX(n)) are stored in the memory unit 182, respectively(feedback loop FB1). When the n-th loop has been completed, an endingflag is set (step 8).

When detecting the ending flag, by an instruction from the control unit181, the skew calculation unit 185 in the operational processing unit183 calculates skews (step S9). For example, phase differences of theQ_(X), I_(Y), and Q_(Y) ports are obtained using the I_(X) port as areference for number of measurements n respectively, as follows.φ_(IX(n))=0φ_(OX(n))−φ_(IX(n))φ_(IY(n))−φ_(IX(n))φ_(QY(n))−φ_(IX(n))

FIG. 7 shows a diagrammatic illustration plotting the relations betweeneach phase difference of φ_(QX-IX) and φ_(IY-IX) in the Q_(X) port andI_(Y) port using the I_(X) port as a reference and the angular frequency2πf_(max). Here, if a 90-degree error between the I port and the Q portcan be neglected, approximation formulae represented by linear functionsare derived for the Q_(X) port and I_(Y) port respectively, as follows.φ_(QX-IX) =a ₁(2πf)+π/2φ_(IY-IX) =a ₂(2πf)An approximation formula can be similarly derived for the Q_(Y) port asfollows.φ_(QY-IX) =a ₃(2πf)Each gradient of a₁, a₂, and a₃ obtained here represents a skew againstthe I_(X) port.

In this way, if a 90-degree error can be neglected, an interchannel skewcan be detected more simply.

The Second Exemplary Embodiment

Next, the second exemplary embodiment of the present invention will bedescribed. FIG. 8 is a block diagram showing the configuration of anapparatus for detecting interchannel skew in the coherent opticalreceiver 2000 in accordance with the second exemplary embodiment of thepresent invention. The apparatus for detecting interchannel skew in thecoherent optical receiver 2000 includes a coherent optical receiver 200,a test light source 270, and a control block 280 connected to thecoherent optical receiver 200.

The coherent optical receiver 200 has a local light source 210, a90-degree hybrid circuit (90° HYBRID) 220, optoelectronic converters(O/E) 230, analog to digital converters (ADC) 240, and a digital signalprocessing unit (DSP) 250. The control block 280 includes a control unit281, a memory unit 282, and an operational processing unit 283, and theoperational processing unit 283 is provided with a peak detection unit284 and a skew calculation unit 285.

In the coherent optical receiver 200 of the present exemplaryembodiment, the configuration of the digital signal processing unit(DSP) 250 is different from that of the digital signal processing unit(DSP) 150 in accordance with the first exemplary embodiment. The digitalsignal processing unit (DSP) 250 is provided with a complex signalgenerator 252, buffer units (BUF) 253, and an FFT operation unit (FFT)254.

The test light source (TEST) 270 is connected to a signal port 221 ofthe 90-degree hybrid circuit (90° HYBRID) 220, and the local lightsource 210 is connected to a local port 222. Light components outputtedfrom the I_(X), Q_(X), I_(Y), and Q_(Y) ports, which are output ports ofthe 90-degree hybrid circuit (90° HYBRID) 220, are inputted into theoptoelectronic converters (O/E) 230, respectively.

In detecting interchannel skew in the coherent optical receiver 200,first, a continuous wave (CW) light as a test light of a frequency f_(S)(its wavelength is equal to λ_(S)) is inputted from the test lightsource 270 into the signal port 221. Here, a wavelength tunable lightsource can be used for the test light source 270. On the other hand, aCW light as a local light of a frequency f_(O), (its wavelength is equalto λ_(O)) is inputted from the local light source 210 into the localport 222. The test light of frequency f_(S) and the local light offrequency f_(O) interfere in the 90-degree hybrid circuit 220, and beatsignals of a frequency f_(IF)=|f_(S)−f_(O)| are outputted. Here, thebeat signals outputted from the I_(X), Q_(X), I_(Y), and Q_(Y) ports arerepresented by the above-mentioned formulae from (7) to (10) as is thecase in the first exemplary embodiment.

These beat signals are converted into electrical signals by theoptoelectronic converters (O/E) 230, quantized by the analog-to-digitalconverters (ADC) 240, and then inputted into the digital signalprocessing unit (DSP) 250, respectively. In the digital signalprocessing unit (DSP) 250, the signals from the I port and the Q portare synthesized, and then processed as complex signals. That is, thecomplex signal generator 252 receives I_(x) and Q_(x), and outputs acomplex signal of E_(x)=l_(x)+jQ_(x). Similarly, it receives I_(y) andQ_(y) and outputs a complex signal of E_(y)=I_(y)+jQ_(y).

These complex signals E_(x) and E_(y) are divided into blocks withrespect to each predetermined processing unit (4096 bits, for example)by buffer units 253, and subjected to an FFT process in the FFToperation unit (FFT) 254. As a result, each of matrices E^_(x)(N) andE^_(y)(N) is obtained as each output of the FFT operation unit 254.Here, “N” represents a point number of FFT and it is equal to a valuefrom 0 to 4095, for example.

In this case, E^_(x)(N) is represented by the following formulae.

$\begin{matrix}{{{\cos\left\lbrack {{\Delta\omega}\left( {t - T_{1}} \right)} \right\rbrack} + {j\;{\sin\left\lbrack {{\Delta\omega}\left( {t - T_{2}} \right)} \right\rbrack}}} = {{\frac{1}{2}\left( {{\mathbb{e}}^{j\;{{\Delta\omega}{({t - T_{1}})}}} + {\mathbb{e}}^{{- j}\;{{\Delta\omega}{({t - T_{1}})}}}} \right)} + {\frac{1}{2}\left( {{\mathbb{e}}^{j\;{{\Delta\omega}{({t - T_{2}})}}} - {\mathbb{e}}^{{- j}\;{{\Delta\omega}{({t - T_{2}})}}}} \right)}}} \\{= {\frac{1}{2}\left\{ {{{\mathbb{e}}^{j\;{\Delta\omega}\; t}\left( {{\mathbb{e}}^{{- j}\;{\Delta\omega}\; T_{1}} + {\mathbb{e}}^{{- j}\;{\Delta\omega}\; T_{2}}} \right)} + {{\mathbb{e}}^{{- j}\;{\Delta\omega}\; t}\left( {{\mathbb{e}}^{j\;{\Delta\omega}\; T_{1}} - {\mathbb{e}}^{j\;{\Delta\omega}\; T_{2}}} \right)}} \right\}}} \\{= {\frac{1}{2}\left\{ {{{\mathbb{e}}^{j\;{\Delta\omega}\; t}P_{1}} + {{\mathbb{e}}^{{- j}\;{\Delta\omega}\; t}P_{2}}} \right\}}}\end{matrix}$where P₁, P₂, and Δω are represented by the following formulae.P ₁ =e ^(−jΔωT) ¹ +e ^(−jΔωT) ²P ₁ =e ^(jΔΩT) ¹ −e ^(jΔωT) ²Δω=2πf _(IF)

Next, the method for detecting interchannel skew in the coherent opticalreceiver in accordance with the present exemplary embodiment will bedescribed. The flow of its process is similar to that in the firstexemplary embodiment, and therefore the following description will begiven also referring to the flowchart shown in FIG. 3. First, afrequency of the test light source 270 is set at a frequency f_(S1) (itswavelength is equal to λ_(S1)) (step S1). Accordingly, a beat signal ofa frequency f_(IF)=|f_(S1)−f_(O) is outputted from each output port ofthe 90-degree hybrid circuit (90° HYBRID) 220.

Next, data capturing process is started (step S2). At that time, thecontrol unit 281 in the control block 280 transmits a data capturesignal to the digital signal processing unit (DSP) 250 (step S3). TheFFT operation unit 254 receives the data capture signal, triggered bythe signal, it performs an FFT process on the data stored in the bufferunits (BUF) 253 at that time, and returns FFT data E^_(x)(N) andE^_(y)(N) to the control unit 281 (step S4). The control unit 281 storesthe acquired FFT data in the memory unit 282 (step S5).

By an instruction from the control unit 281, the peak detection unit 284in the operational processing unit 283 extracts two peak values ofP₁=|E^_(x)(N_(peak1))| and P₂=|E^_(x)(N_(peak2)) from 4096 points of theFFT data E^_(x)(N). And then, the frequencies of ±2πf_(IF) at that pointare derived by calculation (step S6). In FIG. 9, a diagrammaticillustration is shown where E^_(x)(N) are plotted against point numberN. Here, since the FFT data E^_(x)(N) are composed of complex numbers,the vertical axis of the figure represents the magnitude of E^_(x)(N),|E^_(x)(N)|, and the horizontal axis represents the point number N inthe FFT data. As shown in FIG. 9, if |E^_(x)(N)| has peak values at thepoint numbers N_(peak1) and N_(peak2), the peak detection unit 284detects P₁ and P₂. Here, f_(T) representing a sampling frequency in theanalog-to-digital converter (ADC) 240, a frequency interval of the FFTprocess is equal to f_(T)/4096. Therefore, the peak frequencies at thepeaks of E^_(x)(N) are equal to f_(peak1)=N_(peak1) f_(T)/4096 andf_(peak2)=−(4096−N_(peak2)) f_(T)/4096, respectively.

Next, phase information of φ_(IX) and φ_(QX) are derived by calculation.First, the peak value P₁ is given by the following formula.

$\begin{matrix}{P_{1} = {{\mathbb{e}}^{{- j}\;{\Delta\omega}\; T_{1}} + {\mathbb{e}}^{{- j}\;{\Delta\omega}\; T_{2}}}} \\{= {\left( {{\cos\;{\Delta\omega}\; T_{1}} + {\cos\;{\Delta\omega}\; T_{2}}} \right) + {j\left( {{{- \sin}\;{\Delta\omega}\; T_{1}} - {\sin\;{\Delta\omega}\; T_{2}}} \right)}}} \\{= {R_{1} + {jI}_{1}}}\end{matrix}$where R₁ and I₁ are represented by the following formulae.R ₁=cos ΔωT ₁+cos ΔωT ₂I ₁=−sin ΔωT ₁−sin ΔωT ₂Further, the peak value P₂ is given by the following formula.

$\begin{matrix}{P_{2} = {{\mathbb{e}}^{{- j}\;{\Delta\omega}\; T_{1}} - {\mathbb{e}}^{{- j}\;{\Delta\omega}\; T_{2}}}} \\{= {\left( {{\cos\;{\Delta\omega}\; T_{1}} - {\cos\;{\Delta\omega}\; T_{2}}} \right) + {j\left( {{{- \sin}\;{\Delta\omega}\; T_{1}} + {\sin\;{\Delta\omega}\; T_{2}}} \right)}}} \\{= {R_{2} + {jI}_{2}}}\end{matrix}$where R₂ and I₂ are represented by the following formulae.R ₂=cos ΔωT ₁−cos ΔωT ₂I ₂=−sin ΔωT ₁+sin ΔωT ₂By those formulae mentioned above, the following relational expressionsare obtained.R ₁ +R ₂=2 cos ΔωT ₁R ₁ −R ₂=2 cos ΔωT ₂I ₁ +I ₂=−2 sin ΔωT ₁I ₁ −I ₂=−2 sin ΔωT ₂By solving these relational expressions, the phase information of φ_(IX)and φ_(QX) are obtained respectively, as follows.

$\phi_{IX} = {{{\Delta\omega}\; T_{1}} = {\tan^{- 1}\left( \frac{- \left( {I_{1} + I_{2}} \right)}{R_{1} + R_{2}} \right)}}$$\phi_{QX} = {{{\Delta\omega}\; T_{2}} = {\tan^{- 1}\left( \frac{- \left( {I_{1} - I_{2}} \right)}{R_{1} - R_{2}} \right)}}$

In this way, the peak detection unit 284 derives the frequency f_(peak1)and the peak phases φ_(IX) and φ_(QX) at the peak of the magnitude ofthe FFT data E^_(x)(N), and the control unit 281 stores them in thememory unit 282 as a frequency f_(x(1,1)), and phases φ_(IX(1,1)) andφ_(QX(1,1)), respectively (step S7). At that time, the other data of theFFT data E^_(x)(N) can be eliminated.

In order to reduce the influence of a measurement error, the processesfrom step 3 to step 7 are repeated n times, and frequencies f_(X(1,n)),and phases φ_(IX(1,n)) and φ_(QX(1,n)) are stored in the memory unit282, respectively (feedback loop FB1). When the n-th loop has beencompleted, an ending flag is set (step S8).

Next, after changing a frequency of the test light source 270 into afrequency f_(S2) (step S9), the processes from step 2 to step 8 arerepeated again, and then frequencies f_(X(2,n)) and phases φ_(IX(2,n))and φ_(QX(2,n)) are stored in the memory unit 282 (step S7). Whendetecting an ending flag (step S8), a frequency of the test light source270 is further swept (step S9), and then the processes from step 2 tostep 8 are repeated again (feedback loop FB2). By repeating the feedbackloop FB2 m times, frequencies f_(X(m,n)) and phases φ_(IX(m,n)) andφ_(OX(m,n)) are stored in the memory unit 282, respectively. Byperforming similar processes for E^_(y)(N), frequencies f_(Y(m,n)) andphases φ_(IY(m,n)) and φ_(QY(m,n)) are stored in the memory unit 282,respectively.

When the above-mentioned processes have been completed, by aninstruction from the control unit 281, the skew calculation unit 285 inthe operational processing unit 283 calculates skews by a similar methodto that in the first exemplary embodiment (step 10).

As mentioned above, according to the apparatus and the method fordetecting interchannel skew in the coherent optical receiver of thisexemplary embodiment, it becomes possible to calculate skews between theoutput ports and 90-degree errors between I port and Q port. Moreover,by compensating the skew values obtained above in the skew compensationunit of the digital signal processing unit, with which the coherentoptical receiver in accordance with the first exemplary embodiment isprovided, it becomes possible to demodulate signals sufficiently eventhough there arises a skew between the channels, and suppress thedegradation of receiving performance.

The Third Exemplary Embodiment

Next, the third exemplary embodiment of the present invention will bedescribed. FIG. 10 is a block diagram showing the configuration of anapparatus for detecting interchannel skew in the coherent opticalreceiver 3000 in accordance with the third exemplary embodiment of thepresent invention. The apparatus for detecting interchannel skew in thecoherent optical receiver 3000 has a coherent optical receiver 300, atest light source 370, a control block 380, and a sampling oscilloscope390 which are connected to the coherent optical receiver 300

The coherent optical receiver 300 includes a local light source 310, a90-degree hybrid circuit (90° HYBRID) 320, and optoelectronic converters(O/E) 330. The control block 380 includes a control unit 381, a memoryunit 382, and an operational processing unit 383, and the operationalprocessing unit 383 is provided with a peak detection unit 384, a skewcalculation unit 385, and an FFT operation unit (FFT) 386.

In the present exemplary embodiment, the configuration is different fromeach of the first and the second exemplary embodiments in that itincludes the sampling oscilloscope 390 instead of the digital signalprocessing unit (DSP) and the control block 380 is provided with the FFToperation unit (FFT) 386. The sampling oscilloscope 390 is provided withfour-channel analog-to-digital converters (ADC) 391 and memory units392.

The test light source (TEST) 370 is connected to a signal port 321 ofthe 90-degree hybrid circuit (90° HYBRID) 320, and the local lightsource 310 is connected to a local port 322. The beat signals outputtedfrom the output ports of the 90-degree hybrid circuit (90° HYBRID) 320,that is, I_(X) port, Q_(X) port, I_(Y) port, and Q_(Y) port, arerepresented by the above-mentioned formulae from (7) to (10) as is thecase in the first exemplary embodiment.

These beat signals are converted into electrical signals by theoptoelectronic converters (O/E) 330, quantized by the analog-to-digitalconverters (ADC) 391 in the sampling oscilloscope 390, and then waveformdata of I_(x)(N), Q_(x)(N), I_(y)(N), and Q_(y)(N) are stored in thememory unit 392. Here, “N” represents the number of the data and takesthe values from 0 to 4095, for example.

Next, referring to a flowchart shown in FIG. 11, the method fordetecting interchannel skew in the coherent optical receiver inaccordance with the present exemplary embodiment will be described.First, the frequency of the test light source 370 is set at a frequencyf_(S1) (its wavelength is equal to λ_(S1)) (step S1). Accordingly, abeat signal of a frequency f_(IF)=|f_(S1)−f_(O)| is outputted from eachoutput port of the 90-degree hybrid circuit (90° HYBRID) 320.

Next, waveform data is captured in the sampling oscilloscope 390 (stepS2). At that time, the control unit 381 in the control block 380transmits a waveform capture signal to the sampling oscilloscope 390(step S3). And then, the waveform data stored at that time in the memoryunit 392 in the sampling oscilloscope 390 are stored in the memory unit382 in the control block 380 (step S4).

The FFT operation unit (FFT) 386 in the control block 380 performs anFFT process on waveform data I_(x)(N), Q_(x)(N), I_(y)(N), and Q_(y)(N)stored in the memory unit 382 (step S5). And then, it returns theprocessed results of FFT data I^_(x)(N), Q^_(x)(N), I^_(y)(N), andQ^_(y)(N) to the control unit 381. The control unit 381 stores theacquired FFT data in the memory unit 382 (step S6).

By an instruction from the control unit 381, the peak detection unit 384in the operational processing unit 383 extracts the data I^_(x)(N_(max))having the maximum magnitude from 4096 points of the FFT data I^_(x)(N).Then, the frequency f_(max) and the phase φ_(max) at that point arederived by calculation (step S7).

In order to reduce the influence of a measurement error, the processesfrom step 2 to step 7 are repeated n times, and frequencies f_(IX(1,n))and phases φ_(IX(1,n)) are stored in the memory unit 382, respectively(feedback loop FB1). When the n-th loop has been completed, thefrequency of the test light source 370 is changed into a frequencyf_(s2) (step S8), the processes from step 2 to step 7 are repeatedagain, and then frequencies f_(IX(2,n)) and phases φ_(IX(2,n)) arestored in the memory unit 382 (feedback loop FB2). By further sweepingthe frequency of the test light source 370 and repeating the feedbackloop FB2 m times, frequencies f_(IX(m,n)) and phases φ_(IX(m,n)) arestored in the memory unit 382, respectively. By performing similarprocesses for Q^_(x)(N), I^_(y)(N), and Q^_(y)(N), frequenciesf_(QX(m,n)), f_(IY(m,n)), f_(QY(m,n)), and phases φ_(QX(m,n)),φ_(IY(m,n)), and φ_(QY(m,n)) are stored in the memory unit 382,respectively.

When the above-mentioned processes have been completed, by aninstruction from the control unit 381, the skew calculation unit 385 inthe operational processing unit 383 calculates skews by a similar methodto that in the first exemplary embodiment (step S9).

As mentioned above, according to the apparatus and the method fordetecting interchannel skew in the coherent optical receiver of thisexemplary embodiment, it becomes possible to calculate skews between theoutput ports and 90-degree errors between I port and Q port. Moreover,by compensating the skew values obtained above in the skew compensationunit of the digital signal processing unit, with which the coherentoptical receiver in accordance with the first exemplary embodiment isprovided, it becomes possible to demodulate signals sufficiently eventhough there arises a skew between the channels, and suppress thedegradation of receiving performance.

In the above-mentioned exemplary embodiments, the coherent opticalreceiver is provided with the polarization diversity type of 90-degreehybrid circuit. However, the 90-degree hybrid circuit is not limited tothat, a single polarization type of 90-degree hybrid circuit or itscombination can be used.

In addition, although the test light source is connected to the signalport of the 90-degree hybrid circuit and sweeps frequencies in theabove-mentioned exemplary embodiments, but not limited to this, by usinga wavelength tunable laser as the local light source, the wavelength ofthe local light source can be swept with the wavelength of the testlight source constant.

In the description above, the “I port” and the “Q port” mean signalsoutputted from the I port and the Q port, which are physical ports,respectively. However, the “I_(X) port”, “Q_(X) port”, “port I_(Y)port”, and “Q_(Y) port” in the description about the formulae from (7)to (10) represent physical ports, respectively.

In the description about FIG. 12 and FIG. 13, the phrase of “fourchannels” indicates four signal lines between the outputs of the90-degree hybrid circuit 620 and the inputs of the analog-to-digitalconverter 640. And “channel 1 (CH1)” indicates the I_(X) port, “channel2 (CH2)” indicates the Q_(X) port, respectively.

The present invention is not limited to the above-mentioned exemplaryembodiments and can be variously modified within the scope of theinvention described in the claims. It goes without saying that thesemodifications are also included in the scope of the invention.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2010-116878, filed on May 21, 2010, thedisclosure of which is incorporated herein in its entirety by reference.

DESCRIPTION OF THE CODES

100, 200, 300 coherent optical receiver

110, 210, 310 local light source

120, 220, 320 90-degree hybrid circuit (90° HYBRID)

121, 221, 321 signal port

122, 222, 322 local port

130, 230, 330 optoelectronic converter (O/E)

140, 240 analog-to-digital converter (ADC)

150, 250 digital signal processing unit (DSP)

151 skew compensation unit

152 demodulation unit

153, 253 buffer unit (BUF)

154, 254, 386 FFT operation unit (FFT)

170, 270, 370 test light source

180, 280, 380 control block

181, 281, 381 control unit

182, 282, 382, 392 memory unit

183, 283, 383 operational processing unit

184, 284, 384 peak detection unit

185, 285, 385 skew calculation unit

252 complex signal generator

390 sampling oscilloscope

600 related coherent optical receiver

610 local light source

620 90-degree hybrid circuit (90° HYBRID)

630 optoelectronic converter (O/E)

640 analog-to-digital converter (ADC)

650 digital signal processing unit (DSP)

1000, 2000, 3000 apparatus for detecting interchannel skew in coherentoptical receiver

The invention claimed is:
 1. An apparatus for detecting interchannelskew in a coherent optical receiver comprising: a coherent opticalreceiver; a test light source; an analog-to-digital converter; an FFToperation unit; and a control block; wherein: the coherent opticalreceiver comprises a local light source, a 90-degree hybrid circuit, andan optoelectronic converter; the 90-degree hybrid circuit makes testlight from the test light source interfere with local light from thelocal light source, and outputs a plurality of optical signals separatedinto a plurality of signal components; the optoelectronic converterdetects the optical signals and outputs detected electrical signals; thecontrol block comprises a peak detection unit and a skew calculationunit; the peak detection unit calculates a peak frequency and a peakphase at the maximum value in the results of a fast Fourier transformprocess for each of the plurality of signal components; and the skewcalculation unit calculates the difference in propagation delay from thepeak frequency and the peak phase.
 2. The apparatus for detectinginterchannel skew in a coherent optical receiver according to claim 1,wherein the skew calculation unit sets the difference in propagationdelay equal to a gradient of a linear function, by which a differencebetween the peak phase corresponding to each of the plurality of signalcomponents is approximated against the peak frequency.
 3. A coherentoptical receiver with a skew compensation unit for compensating thedifference in propagation delay calculated by the apparatus fordetecting interchannel skew in a coherent optical receiver according toclaim 1, comprising: a local light source; a 90-degree hybrid circuit;an optoelectronic converter; an analog-to-digital converter; and adigital signal processing unit, wherein: the 90-degree hybrid circuitmakes multiplexed signal light interfere with local light from the locallight source, and outputs a plurality of optical signals separated intoa plurality of signal components; the optoelectronic converter detectsthe optical signals and outputs detected electrical signals; theanalog-to-digital converter quantizes the detected electrical signalsand outputs quantized signals; and the digital signal processing unitcomprises the skew compensation unit and a demodulation unit fordemodulating the quantized signals.
 4. The coherent optical receiveraccording to claim 3, wherein the digital signal processing unitcomprises an FFT operation unit for performing a fast Fourier transformprocess on the quantized signals, and the difference in propagationdelay is calculated from the results of the fast Fourier transformprocess.
 5. An apparatus for detecting interchannel skew in a coherentoptical receiver, comprising: a coherent optical receiver; a test lightsourice; an analog-to-digital converter; an FFT operation unit; and acontrol block; wherein: the coherent optical receiver comprises a locallight source, a 90-degree hybrid circuit, and an optoelectronicconverter; the 90-degree hybrid circuit makes test light from the testlight source interfere with local light from the local light source, andoutputs a plurality of optical signals separated into a plurality ofsignal components; the optoelectronic converter detects the opticalsignals and outputs detected electrical signals; the analog-to-digitalconverter quantizes the detected electrical signals and outputsquantized signals; the FFT operation unit performs a fast Fouriertransform process on the quantized signals; and the control blockcalculates a difference in propagation delay between the plurality ofsignal components from the results of the fast Fourier transform processand the peak detection unit detects a plurality of peaks from theresults of the fast Fourier transform process, and calculates a peakfrequency and a peak phase at each of the plurality of peaks; and theskew calculation unit calculates the difference in propagation delayfrom the peak frequency and the peak phase.
 6. The apparatus fordetecting interchannel skew in a coherent optical receiver according toclaim 5, wherein the skew calculation unit sets the difference inpropagation delay equal to a gradient of a linear function, by which adifference between the peak phase corresponding to each of the pluralityof signal components is approximated against the peak frequency.
 7. Acoherent optical receiver with a skew compensation unit for compensatingthe difference in propagation delay calculated by the apparatus fordetecting interchannel skew in a coherent optical receiver according toclaim 5, comprising: a local light source; a 90-degree hybrid circuit;an optoelectronic converter; an analog-to-digital converter; and adigital signal processing unit, wherein: the 90-degree hybrid circuitmakes multiplexed signal light interfere with local light from the locallight source, and outputs a plurality of optical signals separated intoa plurality of signal components; the optoelectronic converter detectsthe optical signals and outputs detected electrical signals; theanalog-to-digital converter quantizes the detected electrical signalsand outputs quantized signals; and the digital signal processing unitcomprises the skew compensation unit and a demodulation unit fordemodulating the quantized signals.
 8. The coherent optical receiveraccording to claim 7, wherein the digital signal processing unitcomprises an FFT operation unit for performing a fast Fourier transformprocess on the quantized signals, and the difference in propagationdelay is calculated form the results of the fast Fourier transformprocess.
 9. A method for detecting interchannel skew in a coherentoptical receiver, comprising the steps of: outputting a plurality ofoptical signals separated into a plurality of signal components bymaking test light from a test light source interfere with local lightfrom a local light source; detecting the optical signals and outputtingdetected electrical signals; quantizing the detected electrical signalsand outputting quantized signals; and calculating a difference inpropagation delay between the plurality of signal components from theresults of a fast Fourier transform process and calculating a peakfrequency and a peak phase at the maximum value in the results of thefast Fourier transform process for each of the plurality of signalcomponents; and calculating the difference in propagation delay from thepeak frequency and the peak phase.
 10. The method for detectinginterchannel skew in a coherent optical receiver, according to claim 9,further comprising: in the step of calculating the difference inpropagation delay, setting the difference in propagation delay equal toa gradient of a linear function, by which a difference between the peakphase corresponding to each of the plurality of signal components isapproximated against the peak frequency.
 11. A method for detectinginterchannel skew in a coherent optical receiver, comprising the stepsof: outputting a plurality of optical signals separated into a pluralityof signal components by making test light from a test light sourceinterfere with local light from a local light source; detecting theoptical signals and outputting detected electrical signals; quantizingthe detected electrical signals and outputting quantized signals; andcalculating a difference in propagation delay between the plurality ofsignal components from the results of a fast Fourier transform processand performing the fast Fourier transform process on a signal resultingfrom synthesizing the quantized signals corresponding to the pluralityof signal components; detecting a plurality of peaks from the results ofthe fast Fourier transform process, and calculating a peak frequency anda peak phase at each of the plurality of peaks; and calculating thedifference in propagation delay from the peak frequency and the peakphase.
 12. The method for detecting interchannel skew in a coherentoptical receiver, according to claim 11, further comprising: in the stepof calculating the difference in propagation delay, setting thedifference in propagation delay equal to a gradient of a linear functionby which a difference between the peak phase corresponding to each ofthe plurality of signal components is approximated against the peakfrequency.